Once-through steam generator recirculating startup system

ABSTRACT

A once-through vapor generator including a startup bypass in which the flow in the bypass is at a reduced pressure. In the bypass, the flow is separated into vapor and liquid streams, the liquid stream being recirculated directly to the suction side of the generator main feed pump. A booster feed pump is provided upstream of the main feed pump and in series therewith. The booster feed pump is operative during startup to provide to the suction side of the main feed pump that makeup flow in addition to the recirculated flow necessary to prevent tube burnout in the generator.

United States Patent Inventor Robert I. Lytle Livingston, NJ.

Appl. No. 2,213

Filed Jan. 12, 1970 Patented Oct 12, 1971 Assignee Foster Wheeler Corporation Livingston, NJ.

ONCE-THROUGH STEAM GENERATOR RECIRCULATING STARTUP SYSTEM 406 S, 406 ST, 448 S, 451 S Primary Examiner-Kenneth W. Sprague Attomeys-John Maier, [11, Marvin A. Naigur and John E.

Wilson ABSTRACT: A once-through vapor generator including a startup bypass in which the flow in the bypass is at a reduced prexure. 1n the bypass, the flow is separated into vapor and liquid streams, the liquid stream being recirculated directly to the suction side of the generator main feed pump. A booster feed pump is provided upstream of the main feed pump and in series therewith. The booster feed pump is operative during startup to provide to the suction side of the main feed pump that makeup flow in addition to the recirculated flow necessary to prevent tube burnout in the generator.

VODE SELEC'OR EME RATUPE l RQOF a :oNvEcncu ENCLOSURE FUR NAGE PASSES ECONOMIZER GLAND SEAL l REGULATOR I F r 'E; 8 DEMI NERin.

ONCE-THROUGH STEAM GENERATOR RECIRCULATING STARTUP SYSTEM The present invention relates to a once-through vapor generator, and in particular to improvements in startup of such a generator.

The present invention will be particularly described with reference to a supercritical once-through vapor generator, although it will be understood that principles of the invention are applicable to a subcritical once-through generator as well.

A once-through generator requires during startup a minimum flow in the heating circuits thereof, for instance, a flow of about 30 percent of full load flow. Since this flow is in a low-enthalpy state, it cannot be transmitted to the generator turbines, and also to certain superheating surfaces of the generator, so that it is bypassed, usually to a heat sump such as the hot well of the condenser for the generator. To conserve on heat, the flow prior to dumping into the condenser frequently is cascaded through a series of high pressure and low-pressure heaters in heat exchange with the generator feedwater. However, only a portion of the heat in the bypass flow can be transferred to the generator feedwater with this arrangement. The remaining heat is dumped to the condenser and lost.

Accordingly, it is an object of the present invention to provide a startup system in which loss of heat during the startup period, for instance to a heat sump such as a condenser, is substantially reduced.

It is also an object of the invention to provide a startup system in which the above is accomplished without added pumping power or other such losses to startup of the generatOl.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawing setting forth in detail a certain illustrated embodiment of the invention, this being indicative, however, of but one of the various ways in which the principles of the invention may be employed.

in such annexed drawing:

The FIGURE is a schematic flow diagram of the generator circuit and startup system in accordance with the principles of the present invention.

In the drawing, there is illustrated schematically a vaporgenerating and turbine installation which includes in series an economizer l2, furnace passes 14, and roof and convection passes 16. Also constituting sections of the generator are a primary and/or platen superheating section 22 and a finishing superheating section 24. During normal operation of the generator, the flow is through the superheating sections to a highpressure turbine 26, exhaust flow from the turbine being reheated at 28 and passed to a low-pressure turbine 30, and from there to condenser 32. Being of the once-through type, the generator heating surfaces are pressurized by a main generator feed pump 34, the flow from the condenser being transmitted first through low-pressure heaters 36, deaerator 38, deaerator storage tank 40, then the main feed pump 34, followed by a pair of high-pressure heaters 42 and the inlet side of economizer 12.

The startup bypass includes a bypass line 44 which is connected between a flash tank 46 and a point 47 on the generator main flow path upstream of the primary superheater 22. A return line 48 is provided between the vapor space of the flash tank and the main flow path, connected to the latter at a point 49 slightly downstream of the point of connection 47. Lines 44 and 48 contain stop valves 50 and 52 to isolate the flash tank from the main flow path. Between the points of connection 47 and 49, there is provided a stop valve 54 in the main flow path,

which, when closed, directs the flow through the flash tank. Between the bypass and the heating surfaces 14, 16 of the generator, there is provided a group of valves 56 in parallel, including four pressure-reducing valves 58 and two remote manual shutoff valves 60. The shutoff valves are operative to direct the flow through the pressure-reducing valves, during at least portions of the startup period, the latter maintaining full operating pressure in the heating surfaces l4. 16 of the generator and a reduced pressure downstream of the valves.

The vapor return line 48 from the flash tank is provided with a number of branch lines 62, 64, and 66, the line 62 leading to condenser 32 via valve 62a; the branch line 64 leading to the gland seal regulator via valve 64a; the line 66 leading to the deaerator 38 via valve 66a.

In accordance with the present invention, the liquid space of the flash tank is provided with a drain line 68 having-branch conduits 68a and 68b, the line 68a leading to the condenser 32 by valve 70. Line 68b is connected directly to the suction side of the main boiler feed pump 34, through valve 72. Also provided in accordance with the present invention is a booster feed pump 76 in the line 78 between the deaerator storage tank 40 and the main feed pump 34. A check valve 80 is provided in the portion 82 of the line 78 on the downstream side of the booster feed pump, and upstream of the point of connection of drain line 68b with the suction side of the main feed During startup of the generator, the drain flow from the flash tank is transmitted primarily through valve 72 of drain line 68b to the suction side of the main feed pump. Valve 70 in drain line 68a is set to regulate either flash tank pressure or water level such that it accommodates only bypass system swell" water, that is excess water in the bypass system caused by the increasing specific volume of the fluid in the recirculating loop. Exceptions to this are only during cycle water cleanup, when there is of necessity. some dumping of heat to the condenser, and also when steam is permitted to bypass the superheater in order to raise final steam temperature at the turbine throttle in a hot restart sequence.

The following example will illustrate operation of the startup system and principles of the invention.

Initially, in the cold startup period, the valves in the bypass system downstream of the flash tank are provided with set points somewhat as shown in table I.

TABLE I Valve p.s.i. Set Point 62a in vapor line 625 to condenser 66a in vapor line 575/50 to deaerator 52 in vapor line 590 to S.l-l. inlet 70 in drain line 550 to condenser The main boiler feed pump 34 is started, and shutoff valve 54 in the main flow line is closed so that the flow is diverted to the flash tank 46. The remote manual valves 60 are closed so that the flow is through the pressure-reducing valves 58, the latter initially being set in full open position. Shutoff valve 72 is open so that the drain flow from the flash tank is into the suction side of the main feed pump 34. The flow at this time is about 10 percent of full-load flow. When the flow has been increased through the main boiler feed pump to about 30 percent of full-load flow, then the burners of the generator are fired at about 15 percent of full load, limited to maintain l,200 F. at the finishing superheater (since the finishing superheater tubes are uncooled, gas temperature across the tubes has to be limited to a predetermined amount).

When the temperature of the flow at the outlet of the roof and convection enclosure tubes reaches about 300 F., the pressure-reducing valves are switched to pressure control to maintain about 2,550 p.s.i. in the upstream surfaces in the case of a subcritical generator or 3,650 p.s.i. in the case of a supercritical unit. Flash tank pressure increases as the fluid expands. Valve 70 limits this pressure to 550 p.s.i., opening if necessary to discharge swell" water. The flash tank remains flooded until the entering fluid reaches saturation temperature. Then flash tank water level lowers gradually. When the level reaches control height, the booster pump '76 is started and placed on level control. Increasing pump flow raises level and decreasing flow lowers it. Valve 70 is removed from pressure control and is sequenced with the booster pump on level control. If the booster pump flow is zero and the level is high, valve 70 will open to lower the level by discharging additional swell water. The pressure increases to about 575 p.s.i., at which point valves 64a and 660 open to release steam to the gland seal regulator and deaerator. Valve 66a has a downstream control responsive to pressure in the deaerator so that as the latter reaches 50 p.s.i., it overrides the upstream control to maintain 50 p.s.i. in the deaerator. The flash tank pressure continues to increase, and at 590 p.s.i., valve 50 opens admitting vapor to the superheaters and warming the turbine.

After initial heating, as the enthalpy in the fluid increases, the turbine is rolled and then loaded. During initial rolling of the turbine, no vapor is dumped to the condenser via valve 620 unless necessary to limit the flash tank pressure to 625 p.s.i.

Following initial rolling of the turbine, the turbine is loaded,

and the load is increased by increasing demand set point and flash tank pressure set point at a progressive rate. Valve 62a is set at 50 p.s.i. above flash tank pressure set point so that only excess vapor is dumped through this valve to the condenser. The load is further increased until the turbine throttle stop valve reaches 100 percent open position at which time the turbine is at about 30 percent load, and the flow from the flash tank to the turbine is the full 30 percent startup flow from the generator heating circuits. In other words, at this point the enthalpy of the fluid entering the flash tank has increased to the extent that it is fully vaporized, and the valves 50 and 52 can then be closed and the bypass system isolated from the main flow path, valve 54 in the main flow path being moved to an open position.

During substantially the full heating cycle of the startup period, drain valve 70 has been opened only to accommodate swells in the flow. The only exception to this as mentioned is during cycle water clean up when the valve is open to transmit flow to the condenser, condenser pump, the filter, and demineralizer into deaerator 38. This is carried out for only a short period of time during the startup period and does not constitute a significant heat loss. Other than that, the liquid drain flow from the flash tank is entirely into the suction side of the main feed pump, so that there is relatively no heat loss by this means.

There is also relatively no heat loss by transmission of vapor flow to the condenser, the vapor flow being substantially entirely to the turbine. However, this may require controlling firing rate to avoid producing excess vapor, necessitating dumping some vapor, and it is a feature of the present invention that the firing rate can be controlled by means of an overriding flash tank pressure signal (signal 84).

During the heating cycle, it is apparent that as the enthalpy of the fluid increases, there will be an increasingly greater vapor flow and reduction in drain flow through valve 72. In order to maintain a 30 percent flow through the main boiler feed pump, to prevent tube burnout, the booster feed pump is used to add that makeup flow which is required. The booster pump is supplied by the deaerator storage tank, which in turn receives exhaust flow from the turbine.

Between about 30 percent to 100 percent load, the load is increased by increasing the firing and pumping rates, and the pressure-reducing station (56) valve port areas. This provides a progressively increasing pressure in the primary and finishing superheaters of the generator.

During a hot restart, essentially the same procedure is followed, except that some vapor may be transmitted to condenser 32 via valve 62a to control the superheater exit temperature.

Advantages of the invention should be apparent. By recirculating the bypass flow directly to the inlet side of the main boiler feed pump, heat losses during startup are avoided to a considerable extent. No limit is placed on heat recovery such as would be imposed were the heat recovery accomplished by means of feedwater heaters.

It is known in a recirculation-type generator to recirculate the bypass flow to the inlet of the generator. However, this conventionally requires a recirculating pump. During normal operation of the generator, the recirculating pump is turned idly representing an operating loss.

In the present invention, the main boiler feed pump is utilized for recirculation, and the booster pump required to provide makeup flow is in series with the main boiler feed pump during normal operation and does not add pumping costs to the operation. As an example, the booster pump may increase the pressure from deaerator storage pressure of about 50 p.s.i. to flash tank pressure of about 600 p.s.i., whereas the main boiler feed pump would increase the pressure then to furnace pressure, for instance, 3,600 p.s.i. for a once-through generator.

If the cycle does not employ a deaerating feedwater heater, the booster pump can be eliminated and its function can be built into the condensate pump.

As a further advantage over the conventional recirculatingtype generator, the recirculated flow in the generator of the present invention is separated into lowand high-enthalpy phases, water and vapor, the latter being sent to the superheater, the former being back to the generator inlet. This means that the inlet flow into the generator will be at a lower fluid inlet temperature, permitting lower tube mass flow rates for equal metal temperatures in the furnace area of the generator.

As a further advantage, saturated vapor is provided to the superheater earlier in the startup period than with equivalent firing rates in a recirculation type generator. For instance, the fluid enthalpy leaving the furnace walls need be only in the order of about 480 to begin supplying saturated vapor into the superheater at 600 p.s.i. If the fluid were at supercritical pressure, the enthalpy would have to be about 1,210 B.t.u.s per pound in a conventional recirculation type generator.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A once-through vapor generator comprising a main flow path including, in series flow relationship, vapor generating and superheating surfaces; a bypass system connected with said main flow path including flash tank means therein; a main boiler feed pump for supplying liquid to said vapor generating surface, and pressure reducing means to reduce the pressure of the flow in said bypass system, the improvement comprising liquid conduit means connecting said bypass system flash tank means with the suction side of main boiler feed pump to transmit liquid flow from said flash tank means to said suction side;

booster feed pump means upstream of the liquid conduit means to transmit makeup flow to the feed pump suction side;

further including branch conduit means to transmit swell liquid from said flash tank means to the generator condenser.

2. The generator of claim 1 wherein the booster feed pump is connected to receive liquid from the generator deaerator storage tank.

3. The generator of claim 2 including a valve in said branch conduit means responsive to liquid level in the bypass system flash tank.

4. The generator of claim 3 including a vapor return line from said flash tank means to at least some of said superheating surfaces, valve means therein, and valve controlled vapor branch lines between said flash tank means and the generator condenser, gland seal regulator and deaerator, the valves in said vapor return line, and branch lines being pressure controlled.

5. The generator of claim 4 including check valve means between said booster pump means and the point of connection of the flash tank means with the suction side of the main boiler feed pump.

6. The generator of claim 3 wherein said valve in the branch conduit means can be made responsive to pressure in the flash tank. l

7. The generator of claim 4 including burners and control means therefor, part of said control means comprising means responsive to the pressure in the flash tank to limit the flash tank pressure to a predetermined amount by controlling firing rate.

8. A once-through vapor generator comprising a main flow path including, in series flow relationship, vapor generating and superheating surfaces; a bypass system connected with said main flow path including flash tank means therein; a main boiler feed pump for supplying liquid to said vapor-generating surface; and pressure-reducing means to reduce the pressure of the flow in said bypass system; the improvement comprising liquid conduit means connecting said bypass system directly with the suction side of said main boiler feed pump to transmit liquid flow from the flash tank means to said suction side;

booster feed pump means upstream of the liquid conduit means to transmit makeup flow to the feed pump suction side;

first valve controlled branch conduit means to transmit liquid flow from the flash tank means to a heat sump, and second valve controlled branch conduit means to transmit vapor flow from the flash tank to the heat sump;

said first and second branch conduit means first and second valves being responsive to liquid level in the flash tank and pressure in the flash tank, respectively.

9. The generator of claim 8 including burner means, control means therefor, said control means being responsive in part to pressure in the flash tank means.

10. The generator of claim 9 wherein said valve means responsive to liquid level in the flash tank can be made responsive to pressure in the flash tank.

11. A method for starting up a once-through vapor generator comprising the steps of essentially all of the flow to a bypass flash tank wherein the flow is separated into vapor and liquid phases;

recycling the liquid phase directly to the generator main feed pump suction side, the vapor phase being returned to the generator main flow path; and

adding makeup flow to said main feed pump suction side to replace the vapor phase separated from the recycled flow.

12. The method according to claim 11 wherein excess swell liquid and excess vapor only are transmitted to the generator heat sump.

13. The method of claim 12 wherein the excess vapor amount is minimized by controlling the firing rate of I the generator. 

1. A once-through vapor generator comprising a main flow path including, in series flow relatIonship, vapor generating and superheating surfaces; a bypass system connected with said main flow path including flash tank means therein; a main boiler feed pump for supplying liquid to said vapor generating surface, and pressure reducing means to reduce the pressure of the flow in said bypass system, the improvement comprising liquid conduit means connecting said bypass system flash tank means with the suction side of main boiler feed pump to transmit liquid flow from said flash tank means to said suction side; booster feed pump means upstream of the liquid conduit means to transmit makeup flow to the feed pump suction side; further including branch conduit means to transmit swell liquid from said flash tank means to the generator condenser.
 2. The generator of claim 1 wherein the booster feed pump is connected to receive liquid from the generator deaerator storage tank.
 3. The generator of claim 2 including a valve in said branch conduit means responsive to liquid level in the bypass system flash tank.
 4. The generator of claim 3 including a vapor return line from said flash tank means to at least some of said superheating surfaces, valve means therein, and valve controlled vapor branch lines between said flash tank means and the generator condenser, gland seal regulator and deaerator, the valves in said vapor return line, and branch lines being pressure controlled.
 5. The generator of claim 4 including check valve means between said booster pump means and the point of connection of the flash tank means with the suction side of the main boiler feed pump.
 6. The generator of claim 3 wherein said valve in the branch conduit means can be made responsive to pressure in the flash tank.
 7. The generator of claim 4 including burners and control means therefor, part of said control means comprising means responsive to the pressure in the flash tank to limit the flash tank pressure to a predetermined amount by controlling firing rate.
 8. A once-through vapor generator comprising a main flow path including, in series flow relationship, vapor generating and superheating surfaces; a bypass system connected with said main flow path including flash tank means therein; a main boiler feed pump for supplying liquid to said vapor-generating surface; and pressure-reducing means to reduce the pressure of the flow in said bypass system; the improvement comprising liquid conduit means connecting said bypass system directly with the suction side of said main boiler feed pump to transmit liquid flow from the flash tank means to said suction side; booster feed pump means upstream of the liquid conduit means to transmit makeup flow to the feed pump suction side; first valve controlled branch conduit means to transmit liquid flow from the flash tank means to a heat sump, and second valve controlled branch conduit means to transmit vapor flow from the flash tank to the heat sump; said first and second branch conduit means first and second valves being responsive to liquid level in the flash tank and pressure in the flash tank, respectively.
 9. The generator of claim 8 including burner means, control means therefor, said control means being responsive in part to pressure in the flash tank means.
 10. The generator of claim 9 wherein said valve means responsive to liquid level in the flash tank can be made responsive to pressure in the flash tank.
 11. A method for starting up a once-through vapor generator comprising the steps of essentially all of the flow to a bypass flash tank wherein the flow is separated into vapor and liquid phases; recycling the liquid phase directly to the generator main feed pump suction side, the vapor phase being returned to the generator main flow path; and adding makeup flow to said main feed pump suction side to replace the vapor phase separated from the recycled flow.
 12. The method according to claim 11 wherein excess swell liquid and excess vapor only are transmitted to the Generator heat sump.
 13. The method of claim 12 wherein the excess vapor amount is minimized by controlling the firing rate of the generator. 